Expanding Dinitrogen Reduction Chemistry to Trivalent Lanthanides via the LnZ₃/Alkali Metal Reduction System:  Evaluation of the Generality of Forming Ln₂(μ-η²:η²-N₂) Complexes via LnZ₃/K

The Ln[N(SiMe₃)₂]₃/K dinitrogen reduction system, which mimicks the reactions of the highly reducing divalent ions Tm(II), Dy(II), and Nd(II), has been explored with the entire lanthanide series and uranium to examine its generality and to correlate the observed reactivity with accessibility of divalent oxidation states. The Ln[N(SiMe₃)₂]₃/K reduction of dinitrogen provides access from readily available starting materials to the formerly rare class of M₂(μ-η²:η²-N₂) complexes, {[(Me₃Si)₂N]₂(THF)Ln}₂(μ-η²:η²-N₂), 1, that had previously been made only from TmI₂, DyI₂, and NdI₂ in the presence of KN(SiMe₃)₂. This LnZ₃/alkali metal reduction system provides crystallographically characterizable examples of 1 for Nd, Gd, Tb, Dy, Ho, Er, Y, Tm, and Lu. Sodium can be used as the alkali metal as well as potassium. These compounds have NN distances in the 1.258(3) to 1.318(5) Å range consistent with formation of an (NN)^2- moiety. Isolation of 1 with this selection of metals demonstrates that the Ln[N(SiMe₃)₂]₃/alkali metal reaction can mimic divalent lanthanide reduction chemistry with metals that have calculated Ln(III)/Ln(II) reduction potentials ranging from −2.3 to −3.9 V vs NHE. In the case of Ln = Sm, which has an analogous Ln(III)/Ln(II) potential of −1.55 V, reduction to the stable divalent tris(amide) complex, K{Sm[N(SiMe₃)₂]₃}, is observed instead of dinitrogen reduction. When the metal is La, Ce, Pr, or U, the first crystallographically characterized examples of the tetrakis[bis(trimethylsilyl)amide] anions, {M[N(SiMe₃)₂]₄}^-, are isolated as THF-solvated potassium or sodium salts. The implications of the LnZ₃/alkali metal reduction chemistry on the mechanism of dinitrogen reduction and on reductive lanthanide chemistry in general are discussed.